Deposition in Japan

Before mentioning the characteristics of Kuroko and epithermal vein-type deposits in Japan, it is worthwhile to briefly describe the metallogeny, geology, geophysics, and tectonic situations of the Japanese Islands. 

Figure 1.2. Distribution of the stratiform Cu-sulfide and chert-hosted Mn deposits in Japan (Sato and Ka.se, 1996). MTL Median Tectonic Line TTL Tanakura Tcetonic Line ISTL Itoigawa-Shizuoka Tectonic Line BTL Butsuzo Tectonic Line.

Major epithermal vein-type deposits in Japan are base-metal type and precious-metal type which are classified based on the ratios of base metals and Au and Ag which have been produced during the past (Table 1.2). 

Estimated total productions of Au, Ag and other metals and Ag/Au total production ratio (Ag/Au, by weight ratio) from the individual vein-type and disseminated-type deposits in Japan (Shikazono, 1986). Type 1-A gold-silver-rich deposits, Type 1-B base-metal-rich deposits, Type 2 disseminated-type deposits... 

Most of epithermal precious-metal vein-type deposits in Japan can be classed as adularia-sericite-type, and low sulfidation-type. Very few hot spring-type deposits (quartz-alunite-type, high sulfidation-type) are found in the Japanese Islands. A summary of various characteristic features of adularia-sericite type (low sulfidation-type) is given mainly in section 1.4. 

DISTRIBUTION OF KUROKO-TYPE MASSIVE SULFIDE DEPOSITS IN JAPAN... 

Generally, tetrahedrite-tennantite composition from Kuroko deposits is characterized by high Zn content, low Fe content, high Cu content, and low Ag content compared with those from vein-tyjre deposits in Japan (Fig. 1.16). Rarely, it contains Hg up to 1 wt% (Ishizuka and Imai, 1998). 

Figure 1.17. Frequency histogram for the Ag content of electrum from Kuroko deposits in Japan (Shikazono...

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

Orebody zoning (Park and Macdiarmid, 1963) is observed in Cu-Pb-Zn deposits. For example, in Osarizawa deposit, which is one of the largest Cu-Pb-Zn deposits in Japan, ore metal zoning from deeper to shallower parts is Cu —> ZnPb —> AuAg. 

Among the epithermal vein-type deposits in Japan, four major types of hydrothermal alteration ean be diseriminated. They are (1) propylitie alteration, (2) potassic alteration, (3) intermediate argillic alteration, and (4) advaneed argillic alteration. The definitions of these types of alteration are mainly based on Meyer and Hemley (1967) and Rose and Burt (1979) who elassified the hydrothermal alteration in terms of alteration mineral assemblages. 

The area of the potassic alteration is not wide, compared with the propylitically altered area. The width of potassic alteration zone away from the vein is generally within several tens of meters (ca. 50 m) (Shikazono and Aoki, 1981 Imai, 1986). The potassic alteration is usually found in the intermediate vicinity of the vein in the epithermal deposits in Japan. Thus it is evident that this type of alteration occurs genetically related to the ore deposition. 

Meyer and Hemley, 1967). However, such lateral and concentric zonation has not been reported from the epithermal vein-type deposits in Japan. Montmorillonite-rich and silica-rich zones exist in the upper part of the Au-Ag veins such as the Seigoshi and Takadama (Nagasawaet al., 1981). 

The Fe +/Mg and Fe- /Fe values of chlorite from Kuroko deposits and Neogene Cu-Pb-Zn vein-type deposits differ greatly (Fig. 1.83). Chlorite from Kuroko deposits contains lower Fe +/Mg and higher Fe /Fe " " values than this from the Neogene vein-type deposits in Japan. The most likely explanation for these differences is that these two types of deposit formed at different states of oxidation, although other... 

Consequently, the composition of chlorite in the discharge zone depends largely on the chemical nature of fluids (factors such as Fe "/Mg, SO /H2S, pH, aj 2+) and temperature. Movement of fluids may also be an important cause for the variability in the ratio of Fe " to Mg in hydrothermal chlorite. Wide compositional variations in chlorite from the hydrothermal ore deposits in Japan, including Kuroko and Neogene Cu-Pb-Zn vein-type deposits, are considered to reflect the variable chemical nature of ascending ore fluids and fluids that mix with ascending ore fluids at discharge zone. 

Numerous geochemical data (fluid inclusions, stable isotopes, minor elements) on the epithermal vein-type deposits in Japan are available and these data can be used to constrain geochemical environment of ore deposition (gas fugacity, temperature, chemical compositions of ore fluids, etc.) and origin of ore deposits. 

Substantial amounts of homogenization temperature data on the Neogene vein-type deposits in Japan are available (e.g., Enjoji and Takenouchi, 1976 Shikazono, 1985b) and they are summarized in Fig. 1.87. 

Figure 1.87. Summary of filling temperatures of fluid inclusions from Neogene vein-type deposits in Japan. Solid circle represents average filling temperatures of fluid inclusions for individual deposits (Shikazono, 1985b).

Figure 1.89. Activity of 82(052 )-temperature diagram showing possible as and temperature ranges for epithermal Au disseminated-type (hot spring type), epithermal Au-Ag vein-type and epithermal base metal vein-type deposits in Japan (Shikazono 1986 Shikazono and Shimizu, 1988b).

Shikazono (1985b) summarized the assemblage and mode of occurrence of common gangue minerals from more than 70 Neogene epithermal vein-type deposits in Japan. 

SD and S O. 8D and of the ore fluids responsible for epithermal Au-Ag and base-metal vein-type deposits in Japan have been estimated from analyses of fluid inclusions (Hattori and Sakai, 1979) and minerals (Watanabe et al., 1976). These data are shown in Fig. 1.103. 8D values of ore fluids for epithermal Au-Ag vein-type deposits are similar to those of present-day meteoric water values. 8D values of epithermal ore fluids for base-metal vein-type deposits are slightly higher than those of epithermal Au-Ag vein-type deposits. This may be due to the boiling of epithermal base-metal ore fluids and involvement of seawater. 

Figure 1.103. 8D and 8 0 of ore fluids responsible for epithermal Au-Ag vein-type deposits in Japan (Hattori and Sakai, 1979 Imai et al., 1998). S.W. seawater, M.W. line meteoric water line, KK Kushikino, SG Seigoshi, YUG Yugashima, TK Takatama, FUK Fuke, YN Yatani, KN Kanisawa (Yatani), HK Hishikari. 

Figure 1.106. S 0-8 C of carbonates from Neogene vein-type deposits in Japan (open circle = calcite solid circle = rhodochrosite and Mn-calcite solid triangle = dolomite cross = siderite) (Shikazono, 1989).

Figure 1.114. Distributions of major Green tuff-type (solid circle) and the Non-Green tuff-type (open circle) Au-Ag vein-type deposits in Japan (Shikazono, 1999b).

The REE characterics of calcite from the Au-Ag type are variable. For example, calcites from Sado Au-Ag vein, one of the largest Au-Ag deposits in Japan have both signatures of meteric water and magmatic (or igneous) contributions. Positive Eu anomaly is only found in caleite containing low REE from Au-Ag type (Seigoshi deposit) (Shikazono, unpublished). 

As noted already, epithermal vein-type deposits are classified primarily on the basis of their major ore-metals (Cu, Pb, Zn, Mn, Au and Ag) into the gold-silver-type and the base-metal-type. Major and accessory ore-metals from major vein-type deposits in Japan were examined in order to assess the possible differences in the metal ratios in these two types of deposits (Shikazono and Shimizu, 1992). Characteristic major ore-metals are Au, Ag, Te, Se and Cu for the Au-Ag deposits, and Pb, Zn, Mn, Cu and Ag for the base-metal deposits (Shikazono, 1986). Accessary metals are Cd, Hg, Tl, Sb and As for the Au-Ag deposits and In, Ga, Bi, As, Sb, W and Sn for the base-metal deposits (Table 1.22, Shikazono and Shimizu, 1992). Minerals containing Cu, Ag, Sb and As are common in both types of deposits. They are thus not included in Table 1.22. 

Accessory metals from vein-type deposits in Japan (Shikazono and Shimizu, 1992)... 

Figure 1.133. The normal order in other epithermal gold deposits in Japan (Nagayama, 1993a).

Minor elements associated with the vein-type and Kuroko deposits are different. Characteristic minor elements concentrated to the ore deposits are Se, Te, Hg, As, Sb and Bi in the Au-Ag vein-type deposits, Ag, Bi, As, Sb, Sn, W and Mo in the base metal vein-type deposits, and Au, Ag, Sb, As, Mo and Bi in the Kuroko deposits. This difference in minor elements is consistent with that found in the other epithermal vein-type deposits in Japan (Shikazono and Shimizu, 1992). 

This spatial difference is consistent with the distribution with that of hydrothermal deposits of middle Miocene (Kuroko and polymetallic vein-type deposits in Japan). 

Figure 1.170. Diagram showing the octahedral composition of chlorites from the subvolcanrc hydrothermal deposits, propylite, and Kuroko deposits in Japan (Nakamura, 1970). Chlorite occurring as a gangue mineral in the subvolcanic hydrothermal deposits Nos. 1, 2, 3 and 4 Chlorite from the Ashio copper mine. Nos. 5, 6, and 7 Chlorite from the Kishu mine. No. 8 Chlorite from the Arakawa mine. Nos. 9 and 10 Chlorite from the Ani mine. No. 11 Chlorite from the Osarizawa mine. Chlorite from the so-called propylite No. 12 Chlorite from the Yugashima mine. No. 13 Chlorite from the Budo mine. Chlorite from the Kuroko deposits No. 14 Chlorite from the Wanibuchi mine.

Microprobe analyses of sphalerite indicate that sphalerite from the Tsugu deposit is markedly higher in iron than that from the epithermal Au-Ag vein-type deposits in Japan. 

The ranges of /sj and temperature for epithermal Au-Ag vein-type deposits in Japan have been clearly defined based on the chemical composition of sphalerite and electrum, and homogenization temperatures of fluid inclusions (Shikazono, 1985d). Values of /s for the Tsugu deposit are lower than the typical ranges of values for the epithermal Au-Ag vein-type deposits in Japan (Fig. 1.176). Such a low f 2 is in accord with the high Hg content of electrum in the Tsugu deposit. 

Figure 1.174. Frequency (number of analyses) histogram for Ag (atomic %) of gold from the Tsugu deposit (solid) and epithermal gold-silver vein-type deposits in Japan (open). I sample I II sample II. Data sources Tsugu deposit (Shikazono and Shimizu, 1988b) epithermal gold-silver vein-type deposits in Japan (Shikazono 1981, 1986 Shikazono and Shimizu, 1988b).

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